Heretofore, a metallic carrier for carrying an exhaust gas cleaning catalyst has been generally called a metallic honeycomb structure, which is formed by stacking flat steel plates and corrugated steel plates in alternation, and winding the stack into a cylindrical form, the individual layers being spot welded or soldered.
For example, Japanese Patent Publication Laid-open 54-13462/1979 describes a method in which flat steel plates and corrugated steel plates are integrated with each other by spot welding techniques such as electron beam welding or laser beam welding. However, spot welding cannot weld all portions in the axial direction of the honeycomb structure, but welding is achieved only at limited portions in the vicinity of the end faces of the honeycomb structure.
Furthermore, Japanese Patent Publication 63-44466/1988 discloses a method for forming a honeycomb structure by soldering. Soldering means includes a method in which a soldering material is previously inserted between flat steel plates and corrugated steel plates before stacking and winding the plates, a method in which these plates are wound and then coated with a soldering material, followed by melting the soldering material, and a method in which the steel plates, stacked and wound, are dipped in a mixture of a soldering material and a liquid binder. However, in these cases, as in spot welding, soldering is generally performed at limited portions of the end faces of the honeycomb structure because a catalyst must be carried on the steel plates of the honeycomb structure.
Thus, either spot welding or soldering can join only the end faces of the honeycomb structure, and the joining structure has tended to be insufficient.
The inventors subjected honeycomb structures of 200 mm in outer diameter and 150 mm in length, with the end faces spot welded or soldered, to a pushing-in test to measure the strength. The spot welding was performed for portions 5 mm from both ends of the metallic carrier, and the soldering was performed for portions 20 mm from both ends by dipping, followed by heating to achieve soldering, each subjected to the tests. As a result, spot welding displayed a strength of about 500 kg, and soldering displayed a strength of 1.5 tons. The fracture strength (joining strength) of the metallic honeycomb was measured, as will be described later, by pushing the structure into a 105 mm diameter hole.
Observing the fracture condition, it has been hypothesized that in spot welding, concentration of stress tends to occur on the welded portion due to its small joining area, and in soldering, the range of optimum clearance is small and soldering does not function as an effective joining in some portions. Especially, in soldering, only 80% of the portions to be joined achieve a contact, and the remaining 20% produce gaps. This is considered to be due to the fact that crests of conventional corrugated plates are not always flat and tend to be in insufficient contact with flat plates, and the corrugated plates and flat plates contacted at room temperature tend to separate from each other producing gaps due to thermal expansion because these plates are heated to a high temperature of about 1,200.degree. C. during soldering.
Recently, however, with increasing joining strength requirements for metallic carriers, it has become difficult to achieve sufficient strength only by welding or soldering. In particular, there is a problem in that the joined portion tends to undergo a fracture, resulting in a "telescoping phenomenon" because the converter is located in the vicinity of the engine to improve the effects of the catalyst and tends to be affected by high-temperature exhaust gas and vibrations of the engine.
The inventors have conducted intensive studies for a strong joining method which does not undergo telescoping phenomenon even under such severe conditions, and achieved the present invention. Thus, it is a primary object of the present invention to provide a strongly joined metallic carrier which does not undergo a telescoping phenomenon even when subjected to heavy heat cycles and gas pressures, and a production method thereof.